Synthesis and Structural Dynamics of Five-Coordinate Rh(III) and Ir(III) PNP and PONOP Pincer Complexes

[Image: see text] The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of PNP (2,6-(tBu(2)PCH(2))(2)C(5)H(3)N) and PONOP (2,6-(tBu(2)PO)(2)C(5)H(3)N) pincer ligands are described: [M(PNP)(biph)][BAr(F)(4)] (M = Rh, 1a; Ir, 1b; biph = 2,2′-biphenyl; Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) and [M(PONOP)(biph)][BAr(F)(4)] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the (1)H NMR time scale (ΔG(⧧) ca. 60 kJ mol(–1)) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (ΔG(⧧) ca. 10 kJ mol(–1)). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperature-dependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives.